Analysis of dosimetric differences between dose-to-water and dose-to-medium calculations for electron beams

In cancer radiation therapy, the conventional approach has been to calculate and report dose in terms of dose-to-water. With the introduction of commercial Monte Carlo based treatment planning systems, it is now possible to calculate and evaluate dose distributions in terms of dose-to-medium, as opposed to the traditional dose-to-water, of conventional planning systems. The two calculation approaches are conceptually different and the method used has an impact on the reported doses to different organs, such as bone and lung. For electron beams, differences between the two methods have been reported to exceed 10% in cases with hard bone. These differences are greatest in materials whose electron densities are furthest from water, such as bone and lung. This has raised the question of which approach is more appropriate. An AAPM task group has recently recommended that both options should be available in commercial software, but as of yet, there is no consensus which approach should be used. There is a need for systematic analysis of clinical data that would help with the understanding of differences between the Dm and Dw approaches, which in turn could lead to a more accurate evaluation of treatment outcomes. The purpose of this study is to investigate the dosimetric differences between plans calculated using the Dm versus Dw approach for clinical breast, and head and neck cases treated with electron beams. The analysis included plans for epoxy-resin-based phantoms containing hard bone, lung, and air heterogeneities, as well as a retrospective study breast, and head and neck cancer patients treated with electron beams. In some cases, where hard bone was present, differences between Dm and D w exceeded 10%. Differences were consistent with the water-to-tissue stopping power.